This invention relates to a high-frequency heating apparatus for performing dielectric heating in accordance with a cavity resonance principle employing high-frequency electromagnetic waves.
As methods of effecting a uniform heating of a heating load in accordance with a cavity resonance (heating chamber) principle employing high-frequency electromagnetic waves, there are known a stirrer system which is adapted to stir the high-frequency magnetic waves, a turntable system which rotates the heating load which is carried on a turntable, and a rotary antenna system wherein a high-frequency electromagnetic energy radiator is rotated. Of these diverse systems, the rotary antenna system has been used the most frequently in compact-type household microwave ovens, for the antenna occupies only a limited space within the heating chamber so as to leave a large effective heating space. The rotary antenna having a substantially L-configuration is disclosed in U.S. Pat. No. 4,316,069 and causes the heating chamber to be excited mainly by a vertical electric field from the center of rotation and a revolving horizontal electric field. Therefore, it is the horizontal electric field that contributes to uniform energy distribution. Since the excitation by the horizontal electric field is liable to vary greatly with a horizontal or flat load, the degree of uniformity of electric field distribution is altered a great deal by a variation in the shape of the load in the horizontal plane. In the system described in U.S. Pat. No. 4,028,521 there is no radiation of waves from the rotary shaft but instead a horizontal electric field is radiated from a revolving horizontal antenna, with the result that the degree of uniformity of electric field varies widely with variations in the shape of the load in the horizontal plane.
In the system described in Japanese Patent Laid-open No. 15594/1971 (Tokkai-Sho-56-15594), the radiation of electromagnetic waves from the center of rotation is controlled by adjusting the length of the horizontal antenna connected to the antenna at the center of rotation in order to prevent radiation of waves from the center of rotation. However, since the direction of the electric field radiation during rotation is horizontal, the system has the same disadvantage as that of the above prior art systems.